The present invention relates to high-voltage control circuits; more particularly, the invention relates to a circuit for use in conjunction with an electrostatic paint spray gun or the like, to regulate the voltage and ionization current so as to prevent hazardous ignition.
Electrostatic field generation systems have long been used in the art of pant spraying in order to obtain control over the quantity and quality of coating material which is applied to an article. Such systems typically operate with the article to be coated placed at an electrostatic ground voltage potential, with the electrostatic spray coating apparatus, or portions thereof, being elevated to voltage potentials typically exceeding 60,000 volts (60 Kv). In the early design of such systems, a high-voltage generating system was typically placed remotely from a spray gun, and the high voltage generated thereby was conducted to the spray gun via an insulated, high-voltage cable. The voltage was conducted through the spray gun, and the high-voltage field was generated via a needle electrode proximate the front of the spray gun, wherein the sprayed particles emitted from the spray gun would pass into and through the electrostatic field. The sprayed particles thereby achieved an electrostatic voltage charge which created an attractive force to assist conveying the particles to the grounded article. The value of such systems was apparent in the overall reduction in overspray, because a high percentage of the sprayed particles would become applied to the article itself.
More recent designs for electrostatic spray coating systems have incorporated parts of the high-voltage generating system into the spray gun body itself. For example, U.S. Pat. Nos. 3,731,145, 3,599,038, and 3,608,823 show electrostatic spray coating systems wherein a voltage multiplier is incorporated into the spray gun body, and a low-voltage external power supply provides a relatively low voltage through a wire connected to the spray gun. Such systems have the advantage of eliminating the bulky high-voltage cable coupled between the external power supply and the spray gun, and of reducing the overall size of the power supply requirements for any given electrostatic voltage delivery system.
The most recent improvements in electrostatic spray systems involves the use of an electrostatic spray gun wherein the high-voltage power supply is wholly incorporated within the spray gun body, thus requiring no external wires or power supplies. Such systems are described in U.S. Pat. Nos. 4,290,091 4,377,838 and 4,491,276, which utilize air as an energy source for driving a miniature air turbine contained within the spray gun body. The air turbine in turn drives a generator for developing a relatively low alternating current (AC) voltage, which is coupled into a voltage multiplier circuit for generating the necessary high voltage.
All of the foregoing systems use a capacitor-diode voltage multiplier system, commonly known as a Cockcroft-Walton circuit, wherein a series of cascaded capacitors and diodes are interconnected to form a full-wave voltage doubler, using as many stages as are required to derive the necessary high output voltage from a given low AC input voltage.
It has been a recurring problem in the prior art to develop such high-voltage electrostatic spray coating systems wherein the hazards of fire or explosion are eliminated or at least minimized. The problem arises particularly when volatile spray coating materials are used, such as paints having volatile solvents, coupled with circuits capable of delivering an electrostatic voltage discharge. The energy of an electrostatic voltage discharge may be sufficient to cause ignition of the solvent vapors associated with the spray coating material, which may result in a flash fire or explosion. The condition of a voltage discharge has been noted to occur shortly after a sudden and rapid increase in the high-voltage power supply current, which current is known as the ionization current from the high-voltage electrode.
Some prior art devices have been proposed to solve this problem by limiting the maximum ionization current which is available from the electrode, by the inclusion of a series resistor with the power supply, so as to reduce the electrode voltage directly proportional to the magnitude of the ionization current. Other prior art designs have sought to solve the problem by utilizing various forms of control circuits, the effect of which is to either shut off the electrostatic high-voltage power supply at certain predetermined ionization current levels, or to create a current load line which is not permitted to exceed a critical ionization current level. The critical ionization current level; namely, the ionization current which is required to initiate a voltage discharge, is reasonably predictable for a spray coating system.
A technical report entitled "Investigation of Minimum Corona-Type Currents for Ignition of Aircraft Fuel Vapors," by M. M. Newman and J. D. Robb, published by the National Aeronautics and Space Administration (NASA) as NASA Technical Note D-440, June 1960, has reported that the minimum corona-type current which was sufficient for the ignition of certain aircraft fuels is about 220 microamperes. This report concerned test results for tests on aviation gasoline, various jet engine fuels, under various conditions of temperature and ignition. Although the critical ionization current level is somewhat affected by atmospheric conditions such as temperature humidity, it is believed that the critical ionization current of about 230 microamps may cause ignition of solvent vapors associated with paint spraying, under most operating conditions.